In a typical method of xerographic reproduction, electric charges are deposited on a photoconductive surface by a corona discharge, after which the charged photoconductive surface is exposed to a light pattern to form a latent electrostatic image thereon. This latent image is then rendered visible by applying toner, which may be electrostatically charged powder or the like, directly to the photoconductive surface so that it adheres thereto in the latent image-bearing areas through electrostatic attraction. The resulting visible image is then fixed to a permanent image, as by heating or the like, to fuse it in place either directly on the photoconductive surface or after print-off to a suitable copy sheet, such as paper.
The abrasiveness of toner powder results in wear of the relatively expensive permanent photoconductive layers used in copying machines, thereby degrading the quality of copies and ultimately requiring replacement of the photoconductive layers. Moreover, difficulties are encountered in fully transferring the visible image from the photoconductive surface to the copy and of keeping the toner powder from image-free areas. Gradual toner powder build-up on and around the photoconductive surface also degrades the copy quality, since inadvertent toner transfer to copies causes the copies to appear gray and splotchy in background areas, reducing contrast and definition.
An additional problem with such reproduction procedures is that a separate exposure of the photoconductive surface is needed for each copy, that is, multiple copies cannot be made from a single exposure of the photoconductive surface. In addition, multiple copies bearing two or more different toner colors cannot be made.
Certain newer xerographic processes have been developed to overcome some of the foregoing drawbacks but are usually relatively complicated and are not adapted for use in simple, inexpensive copying machines.
In copending U.S. patent application, Ser. No. 215,873, now U.S. Pat. No. 3,820,985, filed Jan. 6, 1972 by Joseph Gaynor, Terry G. Anderson, Walter Hines and Len A. Tyler, and assigned to the present assignee, an improved simple electrostatic copying process is provided which permits multiple copies from a single exposure and allows multiple color copying. In that process a thin insulating film is disposed on an electrostatically charged photoconductive surface. An electrostatic image induced on the free surface of the film is developed with electroscopic toner which can be transferred to a copy sheet. The photoconductive surface is thus protected from the abrasive effect of the toner particles.
The present invention provides improvements in contrast and resolution over the Gaynor et al process described above and provides all of its advantages and others. In accordance with the present process, or uniformly charges one side of a transparent sheet of insulating material, such as a thin insulating film as described in the aforenoted Gaynor et al application, or an electret, and places the charged side against an electrostatically charged or uncharged photoconductive surface on a suitable substrate to form a temporary composite. The photoconductive surface is then exposed to a light pattern and the free surface of the transfer sheet is developed to provide a visible image corresponding to the light pattern. This image is fixed on the transfer sheet or transferred to a receiving sheet. The transfer sheet should be removed from the photoconductive surface when the subsequent treatment may affect the electrostatic image (e.g. if fixing or transfer is thermal) or for mechanical facility. In the transfer mode, further copies can be made by reapplying the transfer sheet to the photoconductive surface, if it has been removed, and redeveloping the free surface of the transfer sheet when in place on the photoconductive surface.
Real electrostatic images can be provided on the free surface of the transfer sheet by charging it to a constant voltage, as with a constant voltage-variable current corona device, during or after light exposure. The real electrostatic image can be used to provide multiple visible copies of the image without having to recontact the transfer sheet with the photoconductive surface. This is accomplished by placing the real electrostatic image side of the transfer sheet on a grounded conductor bearing a thin blocking layer and developing the opposite surface of the transfer sheet to provide a visible toner image, transferring the toner image to a copy sheet, and repeating the developing and transferring to provide the desired number of copies.
The present process can also be successfully used when the photoconductive surface is extremely light sensitive and has a high dark decay rate. In one method, the photoconductive surfaces and substrates are transparent. After application of a precharged transfer sheet, the composite is simultaneously exposed and developed (from opposite sides of the composite). In another method, the sandwich is simultaneously charged to constant voltage and exposed.
The present methods provide single or multiple copies in one or a plurality of colors from a single exposure. Moreover, the photoconductive surface is fully protected from wear and contrast loss by the toner. Importantly, the copies are full, sharp, clear and of high contrast and resolution. The process can be carried out in a variety of modes to suit individual needs, all of which modes are characterized, in part, by the use of a precharged transfer sheet of thin insulating film or an electret. The photoconductive surface can be precharged or uncharged. Relatively permanent real electrostatic images can be formed and highly light sensitive, high dark decay rate photoconductors having wide spectral sensitivity can be used efficiently. Further features of the present process are set forth in the following detailed description and accompanying drawings.